This invention is directed to an integrated test circuit for display devices, particularly liquid crystal displays.
Liquid crystal television and computer displays are known in the art. For example, see U.S. Pat. Nos. 4,742,346 and 4,766,430, both issued to G. G. Gillette et al. and incorporated herein by reference. Displays of the type described in the Gillette patents include a matrix of liquid crystal cells which are arranged at the crossovers of data lines and select lines. The select lines are sequentially selected by a select line scanner to produce the horizontal lines of the display. The data lines apply the brightness (gray scale) signals to the columns of liquid crystal cells as the select lines are sequentially selected. Each liquid crystal cell is associated with a switching device through which a ramp voltage is applied to the liquid crystal cells in the selected line. Each of the switching devices is held on by a comparator, or a counter, which receives one of the brightness signals to permit the ramp voltage to charge the associated liquid crystal cell to a voltage proportional to the brightness level received by the comparator from the data line.
Liquid crystal displays for television and computer monitors have a large number of columns of liquid crystal cells, for example 1,440. Assuming substantially square liquid crystal cells and a standard television aspect ratio of 4.times.3, a display with 1,440 data lines would have approximately 1,080 select lines. In an acceptable display, none of this large number of lines can include an open. Also, the data lines and select lines are perpendicular and thus cross over one another and must be electrically insulated. Three types of failures which frequently occur during the fabrication of LCD displays are failed data line scanner stages, open data lines and shorts between select lines and data lines. For this reason, there is a need for a circuit for testing for such flaws which is reliable, fast and inexpensive. The present invention fulfills this need by the provision of an integrated circuit which is fabricated onto the display device substrate and which substantially reduces the number of test lines needed for the display device.